Amines, amides, and amino acids
Properties of amines
Amines are basically ammonia (NH3) with one or more of the hydrogens replaced with an alkyl group (-CH3, C2H5 etc). Primary amines are formed when one alkyl group is bound to nitrogen, secondary if you have two groups and tertiary when there are three groups. Quaternary amines are formed when the nitrogen is bound to four alkyl groups (using its lone pair of electrons).
Ammonia has a lone pair of electrons which it can use to bind to a hydrogen ion, forming ammonium. Since ammonia is able to neutralise hydrogen ions, it is a base. The same is true of all amines, since the nitrogen will always have a lone pair of electrons that it can use to form a dative covalent bond with H+.
Forming amines
Amines are made by reacting ammonia with a haloalkane. For example, adding ammonia to bromoethane will produce ethylamine (the alkyl group has been substituted for hydrogen.) Ethylamine can further react to form diethylamine and trimethylamine. Ammonium bromide is formed as the second product. The amines are separated by fractional distillation.
Aromatic amines (amines which contain a benzene ring) are made by reducing nitrobenzene. Nitrobenzene is heated under reflux in the presence of concentrated hydrochloric acid, a tin catalyst and sodium hydroxide. Phenylamine and water are formed as the products of the reaction.
Amides
Amides are compounds that contain a carbonyl group (C=O) next to an amine group (-NH2). In primary amides, the functional group is found at the end of the molecule and the nitrogen is bound to only one carbon atom. In secondary amides, the nitrogen is bound to 2 carbons.
Proteins contain amide bonds (also known as peptide bonds) which are formed when the carboxylic acid group on one amino acid reacts with the amine group on another amino acid.
Amino acids consist of a central carbon atom attached to four different groups: an amine group, a hydrogen atom, a carboxyl group and a variable 'R’ group which is different in each amino acid. They have the general formula RCH(NH2)COOH.
Amino acids can act as both acids and bases because they contain the acidic carboxylic acid group (COOH) and the basic amine group (NH2).
When amino acids react with alkalis, a salt and water are formed, just as in other acid-base reactions.
When amino acids react with acids, the amino group accepts a proton to form a positively charged ion. This forms a salt with the negative non-metal from the acid.
Amino acids (like carboxylic acids) react with alcohols to form esters in the presence of a sulfuric acid catalyst.
Chirality
A chiral carbon is a carbon atom which has four different groups attached. Amino acids (except glycine which has hydrogen as its variable group) are chiral molecules because their central carbon is attached to four different groups.
Chiral compounds can be arranged in two different ways which are non-superimposable mirror images of each other. These are known as optical isomers or enantiomers. Optical isomers rotate plane-polarised light in different directions – one isomer rotates it clockwise and the other rotates it anticlockwise.
Some chiral compounds may have more than one chiral centre. For each chiral centre, two enantiomers are formed, which means that if there are two chiral centres, there will be four optical isomers and if there are three chiral centres, there will be six optical isomers.
Note that if an alkene group is also present you’ll also have to count the E/Z isomers formed!
Worked example – identifying the number of isomers
How many stereoisomers are formed from 3-methylhept-5-en-2-ol, shown below?
First identify how many chiral centres there are (atoms attached to four different groups). In this case, it’s 2.
Multiply by 2 to get the number of optical isomers. 2 x 2 = 4.
Identify the number of double bonds (in which each carbon is attached to 2 different groups) to identify the number of E/Z isomers. Multiply that number by two.
1 double bond (with each C=C attached to 2 different groups) x 2 = 2 E/Z isomers.
Total number of stereoisomers = 6.